The present invention relates to a black roll for optical measurement, a thin film forming apparatus including the black roll, and a thin film forming method using the black roll. In particular, the present invention relates to a black roll for optical measurement improved to eliminate almost all reflection of light from the surface of the black roll, a thin film forming apparatus including the black roll, and a thin film forming method using the black roll.
In recent years, a method of producing various functional film bodies by forming a desired optical thin film on a plastic film substrate becomes widely available. The method requires a technique of measuring optical characteristics of the optical thin film formed on the plastic film substrate in-situ while moving the plastic film substrate. To measure optical characteristics of an optical thin film formed on a plastic film substrate in-situ during operation of forming the optical thin film on the plastic film substrate, a so-called black roll for optical measurement is generally used. The black roll is composed of a rotating cylinder colored in black for eliminating adverse effect of reflection of measuring light from the surface of the rotating cylinder. A plastic film substrate on which an optical thin film has been formed is moved in such a manner as to be in close-contact with the rotating black roll for suppressing a vertical change in the surface of the plastic film substrate, and optical characteristics such as a reflectance of the optical thin film are measured using an optical monitor.
For an optical thin film having a low reflectance, however, the method of using the above related art black roll causes a problem. Since the reflectance of light from the surface of the optical thin film is as low as almost close to zero in a visible light region, the reflectance of the black roll, which is in a range of 0.01 to 0.05% and is varied if the surface of the black roll is uneven, gives a measurement error to the result of measuring the reflectance of the optical thin film. In other words, according to the method using the related art black roll, although it is intended to measure only the reflection of light from the optical thin film formed on the surface of the plastic film substrate in close-contact with the black roll, the measured value contains a small degree of reflection of light from the black roll.
If the reflectance of the black roll is constant, the reflectance of an optical thin film can be accurately obtained by subtracting a previously measured reflectance of the black roll from the result of measuring the reflectance of the optical thin film; however, since the surface roughness of the black roll is large, the reflectance of the black roll is unstable. Also, in the case of measuring the reflectance of the optical thin film in-situ by using an optical monitor, the reflectance data obtained in real time is poor in reliability because of non-uniformity of color on the surface of the black roll. In this way, a deviation may easily occur between a value measured in-situ by using the black roll and a value measured without use of the black roll, and therefore, according to the method using the related art black roll, it becomes difficult to stably measure the reflectance of the optical thin film formed on the plastic film substrate in-situ during operation of forming the optical thin film on the plastic film substrate.